Limit switches for electric heating elements



Aug- 26, 1958 R. w. LANCASTER I 2,849,589

LIMIT SWITCHES FOR ELECTRIC HEATING ELEMENTS Filed July 5, 1957 2Sheets-Sheet 1 INVENTOR. RODNEY W. LANCASTER A 7' TOR/V5 Y 1958 R. w.LANCASTER 2,849,589

LIMIT SWITCHES FOR ELECTRIC HEATING ELEMENTS Filed July 5, 1957 2Sheets-Sheet 2 P040?! Sup 04y INVENTOR.

ROD/V6) IV. Z4/VCA57'EA? LIMIT SWITQHES FUR ELECTRIC IEATING ELEMENTE)Rodney W. Lancaster, Red Bank, N. 3.

Application duty 5, 1957, Serial No. 670,156

11 Claims. (Cl. film-39) This invention relates to a limit switch forelectric heating devices and especially for electric room air heaters.

To date, limit switches of solder type links and thermostats have beenused as protecting devices in electric heaters but such devices areunable to accurately control external radiant energy. The commonly usedsolder fuse is usually located close to the heating element in order tosense the temperature thereof. Since much of the damaging force of anelectric heater is in the form of radiant heat energy, the solder typelinks cannot respond accurately because of the variable mass of thesolder from fuse to fuse, and the discoloration of the surface of thesolder by aging, radiant energy being reflected by some surfaces andabsorbed by others. Another disadvantage of the solder type ling whenapplied to an open coil type heating element is found in that themomentum of the blade of the link when opening the circuit will cause itto swing far enough to contact the heating element itself and shortcircuit the unit. Sometimes the solder holding the blade will onlysoften and allow the blade to move slowly toward the open position butthe circuit is never really opened since the solder drawn like tatfywill sustain the circuit.

It is an object of the instant invention to provide a limit switch whichcan be located other than in close proximity to the heating element.

A further object is to create a limit switch that can be located so asto be more accessible to the user than heretofore possible.

Another object is to provide a limit switch which will give ampleprotection to plastic material when used in some parts of electricheaters.

Other objects of the instant invention will become apparent in thecourse of the following specification.

In the attainment of the aforesaid objectives, subject limit switch ismade in live embodiments, the first three of which are concerned withprotection for an electric heating element that has been incorporated ina room air cooling unit while the remainnig two are concerned with aroom air electric heat exclusively. In the first embodiment, the limitswitch is constituted of a removable fusible link in series in thecircuit of an open helical coil heating element added to a room aircooling unit, the fan of the cooling unit circulating the room airtherethrough. The second embodiment is like the first except that someof the heated room air is concentrated against the removable fusiblelink. In the third embodiment, an open coil heating element isconstricted at one point in lieu of the fusible link of the first twoembodi: ments. The fourth embodiment covers the use of a fusible link inseries in the circuit of an enclosed helical coil heating element in anelectric room air heater rather than the combined cooling and heatingunits of the first three embodiments. The fifth embodiment is similar tothe fourth but with the coil of the heating element constricted at onepoint as a substitute for the removable fusible link of the fourthembodiment.

The invention will appear more clearly from the following detaileddescription when taken in conjunction with the accompanying drawingsshowing by way of example the preferred embodiments of the inventiveconcept.

In the drawings:

Figures 1, 2, and 2a show the first embodiment of the limit switchconstructed in accordance with the principles of this invention, wherelike reference numerals indicate like parts, and in which:

Figure 1 is a perspective view in part section of a room air coolingunit in which the limit switch in the form of a removable fusible linkis connected in series in the circuit of an open helical coil heatingelement added to the cooling unit;

Figure 2 is an enlarged perspective view of the limit switch shown inFigure 1 as viewed from the back; and

Figure 2a is a wiring diagram for the removable fusible link.

Figures 34 show the second embodiment of the limit switch, and in which:

Figure 3 is an enlarged fragmentary side view in part section of theremovable housing for the limit switch shown in Figure 1; and

Figure 4 is a View along the line 4-4 of Figure 3.

Figure 5 shows the third embodiment of the limit switch and is afragmentary perspective view of an open helical coil heating element asegment of which is constricted to replace the limit switch shown in thefirst two embodiments.

Figures 67 show the fourth embodiment of the limit switch and in which:

Figure 6 is a cross-sectional view of an electric fan heater in whichthe limit switch shown in Figure 2 is inserted in series in the circuitof an enclosed coil heating element; and

Figure 7 is a sectional plan view of the electric fan heater shown inFigure 6.

Figure 8 shows the fifth embodiment of the limit switch and is anenlarged detailed view in cross section of an enclosed coil heatingelement with a segment of the helical coil constricted in lieu of thelimit switch shown in Figure 2.

Referring now in greater detail to the first embodiment of the limitswitch shown in Figures 1, 2, and 2a, reference numeral 20 indicates aroom air cooling unit or air conditioner, and 21 the limit switch forthe open helical coil heating element 27 which has been added thereto.

The room air cooling unit or air conditioner 20 is similar to the unitillustrated and described in applicants copending application, SerialNo. 568,007, filed February 27, 1956. However, in the presentapplication an open helical coil heating element 27 has been added tothe cooling unit of the co-pending application. Also, the housing 22 ofthe present application, in contrast with that illustrated and describedin the co-pending application, is made removable by thes crews 23, 24,25, and the like which are threaded into the casing 26 of the coolingunit 20 to facilitate the renewal of the fusible link of the limitswitch when blown.

The open helical coil heating element 27 is placed in front of thesquirrel cage fan 28 which circulates the room air through the casing ofthe air conditioner by drawing it first through the removable filter 29and the protective screen 30 located over the room air entrance. Anyknown means may be used to discontinue the operation of the coolingcomponents of the air conditioner when the unit is being used as aheater.

ported in the air stream by any known electrical and heat resistantdevices such as the ceramic supports 32 which are held by the spaceduprights 33 and 34.

One terminal 41 of the heating element 27 (Figs. 1 and 2a) is connectedby the conductor 55 (Figs. 1, 2, and 2a) to the terminal 46 (Figs. 2 and2a) of the limit switch 21, thence by the removable fusible link 52(Figs. 2 and 2a) to the terminal 4-3 thereof, thence by the conductor55a (Figs. 2 and 2a) to contact 38d (Fig.

2a) of the three-way switch 38, and thence by the conductor 56a (Fig.2a) to one side of the source ofelectrical energy. Another conductor 56connects the'terminal 35 (Figs. 1 and 2a) of the element 27 to theopposite side of the source of electrical energy while the same side ofthe electrical energy source is connected to one terminal of the motor23a (Fig. 2a) which actuates the fan 28 when the three-way switch 38 isthrown to either of the two on positions. The switch 38 in the onposition 38:: connects the free terminal of the motor 280 with the otherside of the source of electrical energy through the conductor 56a. Inthe on position 38g, the switch 38 conects both the free terminal of themotor 280 and the terminal 48 of the limit switch 21 with the opositeside of the source of electrical energy. Such known circuits prevent theoperation of the heating element unless the fan is turned on also.

The limit switch 21 (Figs. 1 and 2) is constituted of a ceramic or otherelectrical and heat resistant base 41 which is secured, for example, tothe inner surface of the front of the removable housing 22 (Fig. 1) byscrews which are first passed through the integrally formed andconstricted ends (Fig. 2) of the base 41 and then threaded into thefront of the housing 22 or by other known means. Unlike the known soldertype links, subject limit switch can be located in proximity to orremote from the heating element 27. The removable fusible link 52 (Fig.2) may be a very short length of the same type of wire as that formingthe open helical coil of the heating element 27 but of less diameter andremovably connected by thumb nuts, as illustrated, between the terminals46 and 48 of the base 41. The shorter the length of the link 52, theless the mass, and the less likelihood of causing a short circuit whenfused.

Referring now to the second embodiment of the limit switch shown inFigures 3 and 4, reference numeral 60 indicates the limit switch whichis secured to the inner surface of the housing 61, the. housing beingremovably secured to a combined room air cooling and heating unit or airconditioner as in the first embodiment.

The limit switch 60 is similar to that of the prevoiusly described firstembodiment except that in the second embodiment there is the addition ofthe hollow conical frustum 63, open at both ends, the end 64 of maximumdiameter being directed toward the room air exit 65 of the unit 62 wherethe exit is protected by the louvers 72. The open end 66 of minimumdiameter of the frustum is substantially centered over the removablefusible link 67 (Fig. 4) of the limit switch 60. Ears which are fastenedto or integrally formed with the periphery of the conical frustum 63 atthe end 64 of maximum diameter are turned, as most clearly shown inFigure 4, and then fastened to opposite sides of the removable housing61 by the screws 7 and 71 or by any other known means.

The frustum or funnel 63 is actually a magnifying means to establishmore air density over the link 67 and is especially practical forgeneral application since a small change in air density over the heatingelement, which in some cases might allow the element to operate at toohigh a temperature, yields a more sizable air change at the location ofthe link 67. This magnifying feature allows the designer more latitudein selecting the link for any given set of conditions, either excessiveamperage or insufiicient air flow.

In the third embodiment shown in Figure 5, reference numeral 80indicates an open helical coil heating element. As in the previouslydescribed embodiments, the heating element is supported in a room aircooling unit or air conditioner by ceramic supports, one of which isindicated by the reference numeral 81, the ceramic supports beingattached to spaced uprights such as the one indicated by the referencenumeral 82.

The difference between the heating element St) and the previouslydescribed heating elements of the first two embodiments is that asegment of the element 811 is constricted, for example, at 83 whichobviates the need for the removable fusion link of the first twoembodiments.

The terminals, not shown, of the heating element 311 and the motor ofthe electric fan, not shown, are connected by a switch, not shown, to asource of electrical energy and preferably in such a manner that the fanis turned on simultaneously with or prior to the turning on of theheating element as illustrated and described in connection with thefirst embodiment.

When the heating element 80 becomes overheated as, for example when thecirculation of the air through the casing is interrupted by the cloggingof the filter, loose fan blade, locked rotor, or broken belt, the coilof the heating element 80 will break at the constricted segment 83 andopen the circuit through the heating element. To prevent the thus partedends 84 and 85, on opposite sides of the constricted segment 83, fromfalling and perhaps shorting the unit, a washer 94 of mica or the likewith a central opening may be threaded on the helical coil until itrests on top of the ceramic support 81 to hold the upper strand of thecoil 30 as illustrated. In addition, two spaced and like mica supports86 and 87 are provided. The mica support 36 with the oppositely directedlongitudinal slits 3i; and 89 is fitted, as illustrated, between theupper and lower strands of the coil 80 on one side of the constrictedsegment 83. The other mica support 87 with like oppositely directedlongitudinal slits is fitted, as further illustrated, between the upperand lower strands but on the opposite side of the constricted segment83. Of course, the washer 94 is added protection and may be eliminated.

In the fourth embodiment, illustrated in Figures 6 and 7, referencenumeral indicates a fan heater, and 111 the fan heater limit switch.

The fan heater 110 is constituted of the casing 112 with room airentrances 113 (Fig. 6), and the louvers 113 which form a protection forthe room air exits, the :air being circulated through the entrances andout of the exits by the electric fan 115.

Above the fan 115 is an enclosed coil heating element 116 constituted ofthe usual outer sheath 151, as shown in Figure 8, around a known helicalcoil 152 held in position therein by an insulating material, such asmagnesium oxide, in a known manner. in each end of the enclosed coilheating element 116 is an electrode 118 or 119 (Fig. 7), the inner endof each of which is electrically connected to an end of the helical coiland then insulated electrically from the outer sheath 151 (Fig. 8) byany suitable means. The enclosed coil heating element 116 is supportedin the casing 112 of the fan heater by the spaced supports 120, 121,122, 123, and 124 (Fig. 7) and is protected against accidental contactby the upwardly spaced screen 126.

The limit switch 111 is substantially the same as the similar componentillustrated and described in the first embodiment, that is it has aninsulated base 41 (Fig. 2), and terminals 46 and 48 between which is theremovable fusible link 52. One electrode 118 (Fig. 7) of the heatingelement 116 is connected in series with the removable fusible link andwith the switch 134 and the conductor 135 to one side of the source ofelectrical energy. The other electrode 119 (Fig. 7) is connected by aconductor 136 to the opposite side of the source of electrical energy.Preferably, the switch 134 has three positions so that it first turns onin a known manner the motor of the fan 115 and then both the motor ofthe fan 115 and the heating element 116.

In Figure 7, the limit switchlllisshown on'the-side of the casing whilein Figure 6 it is shown under the baflle 128, the baffle being rcmovablyinserted over the top of the casing 112 by the screws 129, 130, and thelike. However, since subject limit switch is responsive to its ownradiant energy which is designed to be proportional to that of theelement 116, the location in order to function is immaterial.

Referring now to the fifth embodiment of the. fan heater limit switchshown in Figure 8, referencenumeral 150 indicates the enclosedcoil'heating element.

The enclosed coil heating element 150 is constituted, as in the fourthembodiment, of an outer sheath 151 substantially in the center of whichis a helical coil 152 electrically connected at one endto an electrode153 which is sealed therein by any suitable insulation 154. The helicalcoil 152 is supported in the outer sheath 151 by any known insulatingmaterial.

Unlike the helical coil of the fourth embodiment, however, a segment 156of the helical coil 152 is constricted, and with such construction theenclosed coil heatingelement may be connected to a source of electricalenergy without the use of the removable fusible link of the first,second, and fourth embodiments. Obviously, well known circuits may beused whereby the turning on of the heating element is simultaneous withor follows the turning on of the fan. Of course, an open coil heatingelement could be used in both the fourth and fifth embodiments.

In operation:

A typical table for a copper alloy limit switch would be:

Elapsed time in seconds after fan failure for circuit to break AmperesApplying the above table to a 1,500 watt heater rated at 115 volts, anddevelopming 13 amperes at 115 volts, if the air velocity fell to 0,subject limit switch would terminate the circuit in about 18 seconds.If, however, the voltage was low, for example, around 90 volts, 10.3amperes, the circuit would sustain itself for 26 seconds which would bepermissible since the heater is putting out only 900 watts at 90 voltsor 60% of the rated power of 1,500 watts.

Based upon the above, the following would be the protection afitorded bysubject limit switch for a heater using a heating element rated at 1,500Watts at 115 volts, which is a common rating because of the 15 amperefuse limitation now prevalent:

On fan failure Volts Amperes trip time of circuit in seconds Extremevoltage expected". 125 14. 2 7 Rated voltage 116 13.0 18 Minimum voltageexpected 105 12.0 23

many modifications and changes can be made in the details ofconstruction within the scope of the appended claims.

What is claimed is: p

1. A thermal limit switch in combination with the circuit of anelectrically operated radiant heater comprising a fusible link in thecircuit of the heater, the link measuring the intensity of the heatenergy given off by the heater by sensing its own self-energy as well asthe amperage therethrough.

2. In a heating unit for a fluid, the unit having an electrical heatingelement and means for passing the fluid in heat transfer relationshipwith the element; a limit switch for the element, a source of electricalenergy for the element and switch, and a circuit connecting the switchwith the element and the source of electrical energy, the switchhaving'self-energy proportional to that of the element.

3. In a heating unit for a fluid, the unit having an electrical heatingelement and means for passing the fluid in heat transfer relationshipwith the element; a fusible limit switch for the element, a source ofelectrical energy for the element and switch, and a circuit connectingthe switch with the element and the source of electrical energy, theswitch having a breaking point proportional to the temperature of theelement and. the current in the circuit.

4. In a room air heating unit having a casing, the casing having anelectric fan and heating element, and the fan circulating the airthrough'the casing in heat exchange relationship with the element; afusible limit switch for the element, the switch comprising a removablefusible link, a source of electrical energy for the element and link,and a circuit connecting the link with the elements and the source ofelectrical energy, the link being in series with the element and furtherbeing responsive only to the same air flow or a portion thereof thatcools the element as well as the flow of electrical energy in thecircuit.

5. In a room air heating unit having a casing, the casing having anelectric fan and heating coil, and the fan circulating the air throughthe casing in heat exchange relationship with the coil; a fusible limitswitch for the coil, the switch comprising a constricted segment of thecoil, a source of electrical energy for the fan and coil, and a circuitconnecting the fan and coil to the source of electrical energy, theconstricted segment being responsive only to the same air flow or aportion thereof that cools the element and the flow of electrical energyin the circuit and resisting the flow of electrical energy more than anyother segment of the coil.

6. In a room air cooling unit having a casing with room air entrance andexit and an electric fan circulating the air therethrough; an electricheating element disposed in the casing in the path of the circulatedair, a removable housing disposed over at least a portion of the exit,

-the housing having a heated room air inlet and outlet, a

fusible limit switch for the element disposed in the housing, the switchcomprising a removable fusible link, the link having a breaking pointproportional to the maximum safe operating point of the element andlocated in at least a portion of the air flow, a source of electricalenergy for the fan and element, and means for connecting the fan andelement to the source of electrical energy with the link in series withthe element.

7. In a room air cooling unit having a casing with room air entrance andexit and an electric fan circulating the air therethrough; an electricheating element disposed in the casing in the path of the circulatedair, a removable housing disposed over at least a portion of the exit,the housing having a heated room air inlet and outlet, a fusible limitswitch for the element disposed in the rousing, the switch comprising aremovable fusible link, the link having a breaking point proportional tothe heat energy given oil? by the element and substantially unresponsiveto the heat energy given oil by the element, a source of electricalenergy for the element and link, a circuit connecting the fan andelement to the source of electrical energy with the link in series withthe element, and means for concentrating at least some of the heatedroom air against the link.

8. In a room air cooling unit having a casing with room air entrance andexit and an electric fan circulating the air therethrough; an electricheating element disposed in the casing in the path of the circulatedair, a removable housing disposed over at least a portion of the exit,the housing having a heated room air inlet and outlet, a fusible limitswitch for the element disposed in the housing, the switch comprising aremovable fusible link in series with the element, the link measuringthe intensity of the heat energy given oil. by the element as well asthe amperage therethrough, a hollow conical frusturn with open endsdisposed in the housing, the open end of maximum diameter being directedtoward the exit and the opposite end toward the link, a source ofelectrical energy for the link and element and for the fan, and meansfor connecting the fan and element to the source of electrical energywhereby the element operates only when the fan is in operation.

9. In a room air heating unit having a casing with room air entrance andexit and an electric fan circulating the air therethrough; an open coilheating element disposed in the casing in the path of the circulatedair, a segment of the coil being constricted and measuring the intensityof the heat energy given off by the element as well as the amperagetherethrough, support means for the constricted segment on at least oneside thereof, and means for connecting the fan and coil to the source ofelectrical energy whereby the coil operates only when the fan is inoperation.

10. In a room air heating unit having a casing with room air entranceand exit and an electric fan circulating the air therethrough; anenclosed coil heating element disposed in the casing in the path of thecirculated air, a fusible limit switch for the coil comprising aremovable fusible link in series with the element, a source ofelectrical energy for the link and element and the fan, the linkyielding self-energy proportional to that of the element and furtherbeing of a size to fracture when a predetermined current is attained inthe circuit, and means connecting the fan and element to the source ofelectrical energy whereby the element operates only when the fan is inoperation.

11. In a room air heating unit having a casing with room air entranceand exit and an electric fan circulating the air therethrough; anenclosed coil heating element disposed in the casing in the path of thecirculated air, a fusible limit switch for the coil comprising aconstricted segment of the coil, the segment measuring the intensity ofthe heat energy given on by the element as well as the amperagetherethrough, a source of electrical energy for the fan and coil, andmeans connecting the fan and coil to the source of electrical energywhereby the coil operates only when the fan is in operation.

References Cited in the file of this patent UNITEDSTATES PATENTS1,991,280 Hynes Feb. 12, 1935 2,006,810 Kunz July 2, 1935 2,015,251Browning Sept. 24, 1935

